# [Python-ideas] math.nextafter

Juraj Sukop juraj.sukop at gmail.com
Mon Feb 6 08:42:09 EST 2017

```Do you mean something like:

isclose(f(x), 0.0, rel_tol, abs_tol)

If so, what should `rel_tol` and `abs_tol` be?

On Mon, Feb 6, 2017 at 2:16 PM, M.-A. Lemburg <mal at egenix.com> wrote:

> On 06.02.2017 13:22, Juraj Sukop wrote:
> > On Mon, Feb 6, 2017 at 11:29 AM, M.-A. Lemburg <mal at egenix.com> wrote:
> >
> >>
> >> Juraj: Could you provide some use cases, where such a function
> >> would help in Python applications ? (I can see use cases
> >> written in C, but due to the low level, find it hard to
> >> believe that people would use this at the Python level)
> >>
> >
> > In my case, `nextafter` would be used to check if a number is close to
> > polynomial zero, e.g.:
> >
> >     def f(x):
> >         return 2.0*x**3 - 3.0*x**2 + 5.0*x - 7.0
> >
> >     # x = 1.4455284586795218
> >     x = 1.445528458679522
> >     # x = 1.4455284586795223
> >     # x = 1.4455284586795225
> >
> >     left = nextafter(x, -float('inf'))
> >     right = nextafter(x, float('inf'))
> >
> >     print((f(left) < 0.0) != (f(x) < 0.0) or (f(x) < 0.0) != (f(right) <
> > 0.0))
>
> Isn't this something you can do with math.isclose() ?
>
> This would even give you a predefined error range,
> not a dynamic one.
>
> --
> Marc-Andre Lemburg
> eGenix.com
>
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